Open sensor. I rechecked it in the kitchen last night, lights on, and it worked fine. I'm wondering if the diffuser had anything to do with it. The last one worked fine even with the diffuser spun around over the sensor. I did get some pretty black pictures, so maybe the other strobe wasn't positioned where the light got there on the ones I missed. I'll try it again this weekend and see what happens. Got my Sea & Sea 16mm Nikonos WA lens today that I bought on e-bay (50 pounds+postage) so maybe I'll try them together.